Method and apparatus for controlling a sealed atmosphere



March 22, 1966 R. A. BERGAN 3,241,903

METHOD AND APPARATUS FOR CONTROLLING A SEALED ATMOSPHERE Original FiledAug. 16, 1960 2 Sheets-Sheet 1 VAN DE GRAAFF GENERATOR i i l FIG. I

POWER AMPLIFIER DIFFERENCE AMPLIFIER FIG. 2

PRIMARY POWER SUPPLY INVENTOR R. A. BERGAN ATTORNEY March 22, 1966 R. A.BERGAN METHOD AND APPARATUS FOR CONTROLLING A SEALED ATMOSPHERE OriginalFiled Aug. 16, 1960 2 Sheets-Sheet 2 FILAMENT IONIZATION DEUTERIUM POWERPOWER GAUGE SUPPLY SUPPLY SUPPLY 32 x POWER |4 SUPPLY 4| (PHILIPS 3%POWER (PIRANI GAUGE SUPPLY 1 IONIZATION GAUGE TUBE) -2K.V. \39 PUMPINGFIG. 5 SUPPLY SYSTEM 34 Y FIG. 4

25 FIG. 3 P

INVENTOR.

R. A. BERGAN ATTORNEY United States Patent l 3,241,903 METHQD ANDAPPARATUS FOR CONTROLLING A SEALED ATMOSPHERE Reuben A. Bergan, Houston,Tex., assignor, by mesne assignments, to Dresser Industries, Inc.,Dallas, Tex., a corporation of Delaware Original application Aug. 16,1960, Ser. No. 49,997, new Patent No. 3,123,739, dated Mar. 3, 1964.Divided and this application Jan. 9, 1963, Ser. No. 250,402

1 Claim. (Cl. 316-24) This is a division of the copending applicationSerial Number 49,997, filed Aug. 16, 1960, for Method and Apparatus forControlling a Sealed Atmosphere, Patent No. 3,123,739, issued March 3,1964.

This invention relates to the control of the atmosphere in an electricdischarge tube, and more particularly to methods and apparatus forcontrolling the purity and density of the atmosphere in a sealed orstatic atmosphere ion accelerator tube employing the well knowndeuteriumtritium reaction to produce neutrons.

In radioactivity well logging, it has long been conventional to bombardearth formations surrounding a well with neutrons, and to record as afunction of depth the neutrons scattered by the formations, or tosimilarly record the gamma rays produced in the formations by theneutron bombardment. Originally, the neutron sources in such loggingconsisted of a mixture of radium and beryllium. Although such sourcesare generally satisfactory for some purposes, and in fact are stillwidely used, the gamma rays emitted in large quantity with the neutronsfrequently obscure the measurement of the gamma rays induced in thebombarded formations by the neutrons.

- Moreover, safety and. economy requirements necessarily provide a limitto the intensity of such sources when used for well logging purposes. Inaddition, the relatively low energy of neutrons emitted byradium-beryllium sources limits the depth of formation penetration bythe neutrons, and thus limits the lateral depth of investigation.Moreover, radium-beryllium sources simultaneously emit neutrons ofvarious energies. Since spectral analysis of scattered neutronsreturning from the bombarded formations is best achieved with amono-energetic neutron source, radium-beryllium sources areunsatisfactory for the making of this particularly useful measurement.

To overcome these disadvantages, other natural neutron sources such aspolonium-beryllium, plutoniumberyllium, and actinium-beryllium mixtureshave been developed. Some of these neutron sources are comparativelygamma-free. However, since they are natural sources they cannot beturned off, or pulsed, in any manner except by the interposition ofsubstantial shielding, and thus are subject to the aforementionedlimitations of economy and safety.

It was to avoid these difliculties and limitations that the staticatmosphere ion accelerator was developed and combined with anelectrostatic generator. This device, the principles of which aredescribed in US. Letters Patent No. 2,689,918, issued September 21,1954, to Arthur H. Youmans, provides an almost completely mono-energeticand gamma-free supply of neutrons of about 14 mev. energy. Such a devicecan also be pulsed or turned off as desired. However, the staticatmosphere ion accelerator or accelerator tube, as it is more commonlycalled, is essentially designed and operated as a high vacuum electricdischarge tube. It, therefore, is subject to the operating difficultiespeculiar to this class of electronic equipment, particularly whensubjected to the temperature and pressure extremes often encounteredduring well logging.

Basically, the accelerator tube comprises an ion producing region and anaccelerating region. In the ion pro-- 32 1i ,93 Patented Mar. 22, 1956ducing region a cathode and an anode are disposed in a deuteriumatmosphere having a pressure of the order of l0- mm. Hg. When theelectrostatic generator applies a positive charge of sufiicientmagnitude (usually 1,000 to 2,000 volts) to the ion source to produceionization of the intervening deuterium in the usual manner, a magneticfield is provided. to deflect the electron flow, from cathode to anode,to lengthen the path of travel and thereby greatly increase the extentof ionization. The resulting deuterium ions, being positively charged,are attracted toward the cathode. The cathode, however, beingconstructed of open mesh or screen, is relatively transparent to thedeuterium ions. Upon diffusing through the mesh cathode, the ionsencounter a high voltage accelerating field between the ion producingregion and ground, and are accelerated to high energies before strikinga tritium-impregnated target. The bombardment of the tritium nuclei, bythe accelerated deuterium ions, produces neutrons having approximately14 mev. energy by means of the well known deuterium-tritium reaction.

Since the electrostatic generator provides a constant current flow tothe aforementioned anode, the voltage at the anode required to achievethe sought-for ionization is dependent upon the pressure of theatmosphere to be ionized. Moreover, it is also obvious that anyimpurities in the atmosphere will be also ionized when the electron flowoccurs. Thus, a constant neutron output of high intensity and efiiciencycan only be achieved with an accelerator tube having an atmosphereapproaching optimum pressure and purity. Only a very slight deviationfrom optimum is required to materially affect the performance of theaccelerator tube as a neutron source for well logging purposes.Moreover, the substances constituting impurities are substances such ashydrogen, oxygen, nitrogen, carbon compounds, and water vapor. Many ofthe metals commonly used in the structure of the tube have a particulartendency to absorb (or adsorb) these impurities at certain temperaturesand to desorb them at higher temperatures. Since the range of thesetempera tures is usually within the range of temperatures found inwells, it is often the case that an accelerator tube which operatessatisfactorily at the surface of the earth, will operate imperfectly inthe well. Such a case is especially true for tubes which have beenoperated for extended periods.

This out-gassing phenomenon is, of course, a characteristic of allhigh-vacuum electric discharge tubes, and need not be discussed atlength. However, in addition to the aforementioned impurities tending tooccur in all such tubes, the accelerator tube atmosphere also receivestritium from the target due to the bombardment by the deuterium ions.This tritium is ionized in the ion producing region and the resultingtritium ions are accelerated back into the target. Although they willproduce neutrons if they strike deuterium nuclei in the target, theyrequire much more energy (due to their relatively greater mass) toachieve the required speed. Moreover, since the tritium in the targetgreatly exceeds the deuterium, even after long operation of the tube,relatively few of the tritium ions will strike deuterium nuclei and theresult will be in ineffective except to waste substantial amounts ofpower. Since the size of the power supply is limited in well logging,loss of power is a critical factor. Limitations of space, as well as thefact that all sub-surface equipment must be remotely operated, serves toeliminate the usual methods and apparatus for controlling tubeatmosphere pressure and purity. Moreover, because of the many otheroperating handicaps inherent and always present in well-logging, theusual methods of detecting and identifying the causes of tubemalfunction are also unsuitable. Thus, due to these and otherdifficulties, the benefits to well-logging which are potentiallyavailable with the accelerator tube have not been fully realized.

These disadvantages are overcome with the present invention, and novelmethods and apparatus are provided which establish the atmosphere, inthe accelerator tube, at a constant predetermined level of purity andpressure to thereby greatly improve the operation of the tube and togreatly extend its useful life as a monoenergetic and gamma-free sourceof high energy neutrons. The present invention makes use of theaforementioned aflinity of metal for deuterium and for theaforementioned deleterious atmospheric constituents to remove suchdeleterious constituents from the deuterium atmosphere during tubeoperation, and to add or subtract deuterium to or from the atmosphereduring tube operation to stabilize and control its internal pressure.The present invention also makes use of the finding that the magnitudeof the current flowing through an auxiliary electrode, which electrodeis located intermediate of the heretofore described ion producing regionand the target, is functionally related to the condition of the tubeatmosphere regardless of other tube operating factors. Thus, the currentto this auxiliary electrode may be used to indicate the condition of thetube atmosphere, or may be used to control the operation of apparatusemploying the previously described metallic afiinity for deuterium andthe aforementioned impurities to directly stabilize and control the tubeatmosphere during logging operations. The atmosphere control apparatus,as hereinafter described, may be used independently of the electrodecurrent without departing from the scope of the invention.

The advantages of the present invention are preferably attained bydisposing in the tube atmosphere a single metal sleeve or tube partiallysaturated with a pre-determined amount of deuterium, and, by heating atleast a section of this sleeve to peak temperature of about 900centigrade (depending upon the type of metal composing the sleeve), toremove impurities from the atmosphere and simultaneously to remove oradd deuterium from or to the atmosphere depending upon whether theatmosphere is excessively dense or sparse. Moreover, by continuing theheating of the sleeve at temperatures correlative to atmosphericfluctuations thereafter occurring (due to tube operation or changes inenvironmental conditions), the sleeve will be caused to function as acombination deuterium reservoir-sink to adjust and stabilize theatmospheric pressure as required. The heating means preferably used inthe present invention is an electricallypowered heating filamentdisposed within the sleeve, and thus the heating of the sleeve can beachieved and controlled in several ways. In one very useful form of thepresent invention, as hereinafter described, a constant current ofpre-selected magnitude is applied to the filament to heat at least apart of the sleeve to the minimum temperature at which the metal used tofabricate the sleeve will etficiently absorb impurities occurring in theatmosphere. If the accelerator tube atmosphere has been pre-establishedat approximately the pressure desired, the sleeve will getter (absorband retain) all getterable irnpurities as they appear and will emit ordesorb deuterium, depending upon the atmospheric pressure then existing,and depending upon the temperature of the sleeve. Of course, the sleeveis subject to changes in ambient temperature, as is the tube atmosphereand structure. However, if the filament current is of a sufficientmagnitude, and if the sleeve is heated to a high gettering temperature,only the most extreme changes in ambient temperature will change thesleeve temperature suflicient to affect the sorption process.

In other forms of the present invention, which are especially usefulunder conditions requiring more precise control of the sorption process,the aforementioned auxiliary electrode current may be used, eitherdirectly or indirectly, in selecting the proper filament current. In theaccelerator tube of this particular design there is disposed,intermediate the ionization region and the target, at least onering-shaped electrode having the primary function of suppressingsecondary electron emission from the target. When the atmosphere in thetube is at or near the optimum in operating pressure and purity, andwhen the tube is operating, a flow of current is developed in thesuppressor ring circuit, as it is commonly called, which is small inproportion to the magnitude of the ion circuit. However, if impuritiesappear in the atmosphere, or if the pressure of the atmosphere changesfrom optimum, or if both situations develop, the suppressor current willbe found to change correspondingly as a function of changes in tubeatmospheric purity or pressure. If, instead, the pressure drops, thenthe suppressor current will decrease proportionately. Thus, during theprocessing of an accelerator tube, a comparison of the magnitude of thesuppressor current with a supplemental pressure indication (provided byan ionization gage as hereinafter described) will within practicallimits provide an immediate and accurate indication of the tubeatmosphere. In the preferred forms of the present invention, at least apart of the sleeve is maintained at a gettering temperature during tubeoperation for well logging purposes. When the present invention is usedin the tube during well-logging, all suppressor current variations maybe attributed to changes in pressure after the initial period of tubeclean-up. Therefore, in one form of the present invention the loggingoperator need only adjust the filament current to compensate forpressure deviations indicated by variations in the suppressor current.In another useful form of the present invention, the suppressor currentis itself used to modulate or control the temperature of the sleeve, andthus all necessary adjustments are made continuously, and as required,by changing conditions of tube operation and environment.

As previously stated, not all of the substances constituting impurites,and occurring from time to time in the tube atmosphere, are absor-bablein a manner such that a reduction in sleeve temperature will notre-release them into the tube atmosphere. Although hydrogen and tritiumare absorbable in the sleeve, as hereinafter described, they are alsodesorbable at certain temperatures. In the case of tritium, as beforedescribed, a constant emission into the atmosphere is produced by thebombardment of the target. To overcome this difficulty, inherent in anaccelerator tube requiring a substantially pure deuterium atmosphere, itis preferable that the quantity of deuterium in the sleeve be many timesthe quantity of deuterium in the tube atmosphere. Thus, the mixtures ofdeuterium and tritium absorbed by the sleeve will always (withinpractical limits) consist of a much greater proportion of tritium thanwill the deuterium-tritium mixtures subsequently desorbed by the sleeve.When the atmosphere is held at optimum pressure, and the quantities ofgas absorbed and desorbed are brought to equilibrium, the amount oftritium absorbed will still be many times greater than the amount oftritium desorbed. Thus, the so-called non-getterable impurities will beeffectively removed by the sleeve by dilution, since, if the sleeveinitially contains substantially only deuterium, the Same process willalso remove the hydrogen from the atmosphere. Of course, in those formsof the accelerator tube where the non-getterable gases do not constituteimpurities, such as tubes containing pre-selected mixtures of deuteriumand tritium, the sleeve need contain only enough of the gas mixtureselected to provide a reserve against extraordinary operating demands.

In those forms of the present invention herein described, it ispreferable that the sleeve be formed of titanium since this substancehas been found to have the required ability to getter impurities, and toabsorb and desorb deuterium satisfactorily for the purposes. Zirconiumhas also been found satisfactory, although its sorption characteristicsare somewhat different for the purposes of the present inven-. tion. Ofcourse, the sleeve may be formed of any metal capable of gettering, andalso capable of sorption of hydrogen and its isotopes, to an extentsubstantially greater than the corresponding ability of the metals usedin the structure of the tube. Thus the sleeve may be formed of tantalum,lanthanum, cerium, or uranium without departing from the scope of thepresent invention. Moreover, although it is preferred that the sorptiondevice he in the form of a sleeve having a uniform wall thickness, andhaving the filament disposed inside to apply heat uniformly along itslength, the sleeve may take the form of a rod, a wire, or even a strip,and the heating current may be applied directly to the sorption deviceor to a heating filament adjacently disposed. In addition, if a sleeveis used as the sorption device, it need not necessarily have a uniformwall thickness, since different effects and degrees of sorption andgettering may be obtained with sleeves of different shape withoutdeparting from the scope of the present invention.

Accordingly, it is an object of the present invention to provide novelmethods and apparatus for stabilizing the performance of an electricdischarge tube.

It is also an object of the present invention to provide in an electricdischarge tube having an internal atmosphere composed substantially of apreselected gas selected from the group composed of hydrogen and itsisotopes, in combination therewith a metallic sorption means disposed incontact with said atmosphere and containing said gas to the relativeexclusion of other substances foreign to said sorption means, and meanscausing said sorption means to exchange with said atmosphere relativeamounts of said gas functionally related to the pressure of saidatmosphere.

It is also an object of the present invention to provide in an electricdischarge tube having an internal atmosphere composed substantially of agas selected from the group consisting of hydrogen and its isotopes,said tube having at least one electrode disposed adjacent saidatmosphere and conducting an electric current of a magnitude predictablyrelated to the pressure and purity of said atmosphere, in combinationtherewith a metallic body impregnated with said gas to the relativeexclusion of other substances foreign to said body, said body beingdisposed in a manner such that said body absorbs impurities from saidatmosphere and also exchanges with said atmosphere relative amounts ofsaid gas functionally related to the pressure of said atmosphere andtemperature of said body, and means responsive to the magnitude of saidcurrent for selectively heating at least a part of said body to maintainsaid atmosphere at a preselected pressure substantially to the exclusionof substances other than said gas.

It is also an object of the present invention to provide the method ofmaintaining in an electric discharge tube at a pre-selected pressure asubstantially pure atmosphere composed of a gas selected from the groupconsisting of hydrogen and its isotopes, said tube having in contactwith said atmosphere a metallic tube impregnated substantially with apre-determined amount of said gas to the relative exclusion of othersubstances foreign to said body, said method comprising heating at leasta part of said body to a pre-selected temperature such that said bodypermanently absorbs permanently absorbable impurities from saidatmosphere and absorbs and desorbs said gas respectively from and tosaid atmosphere in substantially equal amounts at said pre-selectedpressure.

It is also an object of the present invention to provide the method ofprocessing an electric discharge tube having an internal atmospherecomposed substantially of a preselected gas selected from the groupcomposed of hydrogen and its isotopes, said method comprisingimpregnating a metallic body with said gas to the relative exclusion ofother substances, heating at least a part of said body in a manner andat a temperature such that said body permanently absorbs permanentlyabsorbable impurities from said atmosphere and such that said body alsoexchanges with said atmosphere relative amounts of said gas functionallyrelated to the pressure of said atmosphere, estab- 6 lishing saidatmosphere at a pre-selected pressure, and sealing said tube when saidexchanges of said gas substantially achieve equilibrium at saidpre-selected pressure.

These and other objects and features of the present invention will beapparent from the following detailed description wherein reference ismade to the figures of the accompanying drawings.

In the drawings:

FIG. 1 is a schematic diagram of the accelerator tube and a continuouscurrent type of electrostatic generator when combined to function as aneutron source.

FIG. 2 is a simplified pictorial view of the accelerator tube, showingthe relative position and appearance of basic structural members, andshowing apparatus comprising one embodiment of the present invention.

FIG. 3 is a pictorial View of one form of the present invention.

FIG. 4 is a more detailed view of certain essential features of one formof the present invention.

FIG. 5 is a schematic diagram of apparatus found useful in processing anaccelerator tube in accordance with the principles of the presentinvention.

In those forms of the invention chosen for the purposes of illustrationin the drawings, FIG. 1 shows functionally an electrostatic generator 2of a type similar to that described in U.S. Patent 2,907,884, issuedOctober 6, 1959, to A. J. Gale. The generator 2 is connected in a mannersuch that a positive charge is applied to the anode 4 of the accelerator3. The positive potential on the anode 4 produces a cold emission fromthe cathode 9 which ionizes the deuterium atmosphere 5 between the anode4 and the cathode. The positively charged deuterium ions are attractedto the cathode 9, and, since the cathode 9 is designed to be relativelytransparent to the ions, pass through the cathode 9 and past anintermediate electrode 6 (which can be the previously mentionedsuppressor rings) to bombard the tritium-impregnated target 8. Thosedeuterium ions which strike tritium atoms in the target 8 may producehigh energy neutrons as heretofore explained.

It is necessary, for maximum operating efiiciency, to establish andmaintain the deuterium atmosphere 5 within a predetermined pressurerange since, if the atmosphere is too dense, ionization will take placein the accelerating region, and if the pressure is too sparse, aninjuriously high potential is required at the anode 4 to achieve thenecessary ionizing current flow between the anode 4 and the cathode 9.Thus, it is preferable that the deuterium atmosphere 5 be maintained ata pressure such that the mean free path of the ions is at least as greatas the distance between the cathode 9 and the target 8.

For conventional equipment of this type, when operated under conditionsmore favorable than those encountered in a borehole, pumping is usuallyemployed to replace deuterium lost in the target 3 and to removedeuterium dc-gassed from the structure (walls, electrodes, etc.) of thetube 3. However, such a remedy is impractical for well-logging purposesdue to the necessity of remote operation and due to the limited spaceand power available. Moreover, as previously explained, the desorptionof deuterium from the tube members is accompanied by desorption of othergases and substances constituting impurities since the power used toachieve this ionization and to accelerate the resulting foreign ions iscompletely wasted insofar as the production of neutrons is concerned.Inasmuch as available power is severely limited in a borehole,competition for the limited power is a critical difficulty.

In FIG. 2, in addition to a pictorial representation of the typicalaccelerator tube 3, there is shown one form of the present invention forestablishing and maintaining a substantially pure deuterium atmosphere 5therein at a pressure less than 10 mm. Hg. The tube 3 componentsdepicted include a socket 13 for connecting the electrostatic generator2 (appearing in FIG. 1 and omitted in FIG. 2 for the sake of simplicity)to the anode 4. The mesh cathode 9 is shown surrounding the anode 4which is usually formed from a single straight wire electrode.Surrounding the cathode 9 are the suppressor rings 11 which serve thecontrol purposes performed in FIG. 1 by the intermediate electrode 6.Surrounding the entire complex of anode 4, cathode 9, suppressor rings11, and intervening deuterium atmosphere 5, is the target 8. The meter15 is included for the purpose of measuring the amount of currentflowing in the suppressor ring 11 circuit (when the tube 3 is beingoperated) and through a load resistor 18. The voltage developed acrossthe resistor 18 is applied to a difference amplifier 20 of standarddesign, and the output signal from the difference amplifier 20 isapplied to a power amplifier 22. A primary power supply 10 is also shownfor the purpose of energizing the difference and power amplifiers 20 and22. In addition, there is shown attached at one end of the tube 3 apressure jacket 14 which houses certain apparatus embodying essentialfeatures of the present invention.

FIG. 3 shows a cutaway pictorial view of the pressure jacket 14, andapparatus therein including a fitting 24 to attach the pressure jacket14 to the accelerator tube 3, a nut 25 having a center aperture anddisposed in fitting 24, two brackets 26, a metallic sleeve 27 preferablycomposed of either titanium or zirconium and supported by the brackets26 more or less concentrically inside the pressure jacket 14. Inside thesleeve 27 is disposed a heating filament 28 which is connectedelectrically, by the insulated conductor 29, to a source of controllablecurrent (not depicted). The heating filament 28 is connected toreference potential (ground) by way of the top bracket 26, the metallicsleeve 27, the bottom bracket 26, the nut 25, the pressure jacket 14,the fitting 24, and the accelerator tube wall (not shown in FIG. 3).

FIG. 4 shows a cutaway pictorial view of the metallic sleeve 27 exposingthe heating filament 28 for the purpose of showing how, in a preferredform of the present invention, the heating filament 28 providessubstantially uniform heating throughout the length of the sleeve 27. Asshown, a sleeve 27 which has a uniform wall thickness throughout itslength will be heated to substantially a uniform temperature throughoutsave for the heat loss at its ends due to the conductivity of themetallic brackets 26.

FIG. 5 shows apparatus for processing an accelerator tube 3 inaccordance with the present invention including a controllable heatingfilament power supply 42 connected by means of the aforementionedinsulated conductor 29 to the heating filament 28 in the pressure jacket14. Also shown is a power supply 39 for energizing the suppressor rings11, a power supply 40 for the ionization gage tube 41 which measures thepressure of the tube 3 atmosphere 5, an external deuterium supply 30 andcontrol valve 31, a Pirani gage tube 36 and power supply 37, a Phillipsionization gage tube 33 and its associated power supply 32', a pumpingsystem 34 and control valve 35, and tube atmosphere control valve 38.

In using the apparatus shown in FIG. 5, it is necessary to first preparethe sleeve 27 shown in FIGS. 3 and 4 by loading it with relatively puredeuterium. This is preferably accomplished by first selecting a tube, ofthe desired metal, in this case titanium or zirconium, having thedesired diameter and wall thickness; and heating it in a vacuum and at atemperature sufficient to drive out many foreign substances (principallyhydrogen). The most satisfactory temperature will preferably be thatjust below the melting point of the metal. The purified titanium tube isthen cooled in the vacuum (the exhausted impurities being pumped away)to about 150 Centigrade. Thereafter, relatively pure deuterium isadmitted to the heating chamber at a pressure of about one atmosphere,and the titanium tube is permitted to cool to room temperature.Thereafter, a sleeve 27 of the proper length may be cut from thedeuterium impregnated titanium tube and then assembled as shown in FIGS.2, 3 and 5, with minimum contamination from the atmosphere.

The first step in the preferred procedure for processing an acceleratortube 3, to establish a proper internal atmosphere 5, comprises reducingthis atmosphere 5 to a pressure of approximately 5X10 mm. Hg by means ofthe pumping system 34 while heating the tube 3 and associated piping toabout 250 Fahrenheit. Next, the accelerator tube 3' and the ionizationgage 41 should be energized, and deuterium from the deuterium supply 30should be admitted to the tube 3 while the pumping system 34 holds thetube 3 atmosphere 5 to a pressure of about 5 10 mm. Hg. This step shouldbe continued for about two or three hours. The anode 4 and cathode 9 maybe energized during this period to draw current and thereby aid cleanupof the tube 3.

At the termination of the clean-up period, the control valve 31 shouldbe closed to cut off the deuterium supply 30, and the tube should bepumped to minimum pressure. Thereafter, the entire system should bepermitted to cool to room temperature, and the pumping system 34 shouldcontinue to pump the tube 3 for about one additional hour. Next, thetube atmosphere control valve 38 should be closed to seal the tube 3,and a pre-selected reference current from the filament power supply 42should be applied to the heating filament 28 in the pressure jacket 14.If the apparatus is constructed in conformance with the geometryhereinafter described, a satisfactory reference current will be acurrent of about 220 milliamps at about 23 volts. Theprecise value ofthe preselected reference current is not critical, but to besatisfactory, the selected current must be such that it is substantiallyless than the current necessary to raise the sleeve 27 to meltingtemperature, but sufficient to heat at least a portion of the sleeve 27to a temperature at which oxygen, nitrogen, and other such impuritiesare continuously gettered (absorbed). Under these circumstances, thepressure of the deuterium atmosphere 5, in the tube 3, will rise wellabove the pre-selected optimum pressure since the heated sleeve 27 willdesorb deuterium in amounts related to the temperature of the sleeve 27and the pressure of the deuterium atmosphere 5. Next, the tubeatmosphere control valve 38 should be opened, and excess deuteriumshould be pumped away until the tube atmosphere 5 stabilizes at thedesired operating pressure while the pre-selected reference current isapplied to the heating filament 28 in the sleeve 27. To test thecondition of the tube atmosphere 5, it is only necessary to close thetube atmosphere control valve 38 and thereafter to observe anydeviations of pressure occurring while the reference current heats thedeuterium impregnated sleeve 27. If deviations occur, the tubeatmosphere control valve 38 should be reopened, and deuterium added toor removed from the tube atmosphere 5 as necessary, while applying thepre-selected reference current to the heating filament 28. T 0 checkwhether the sleeve 27 is functioning adequately, one may re-seal thetube 3 and decrease the filament 281 current in increments of about 5milliamps to achieve a decrease in pressure of the tube atmosphere 5.For the previously mentioned geometry, a filament 28- current ofmilliamps should produce a tube atmosphere 5 pressure of about 5 1O- mm.Hg. If, on the other hand, the filament 28 current is increased to 300milliamps, the tube atmosphere 5 pressure should rise to a pressure ofan order of 1 mm. Hg. If, when the filament 28 current is returned toreference (220 milliamps for the suggested geometry), and the tubeatmosphere 5 pressure returns to the desired operating pressure, thetube 3 has been properly processed and may be sealed by cold welding. Ifthe tube atmosphere 5 pressure fails to return to the pressure desired,add

or remove deuterium as necessary, and then repeat the tests.

Returning to FIG. 2, first assume a de-energized tube 3 which has beenproperly prepared by the foregoing process, and which has an internalvolumetric dimension of approximately one-half of a liter. Next, it acurrent of about 250 milliamps at 28 volts is applied to a heatingfilament 28 disposed as shown in FIG. 4 in a titanium sleeve 27 havingdimensions of approximately 1% in. x /s in O.D.x0.01 in. wall thickness,at least a part (generally the center section) of the sleeve 27 in thepressure jacket 14 will be heated to a peak of around 900 Centigrade,and at least another part or section will be heated to about 650centigrade. This current may be supplied by a controllable source offilament current such as that shown in FIG. 5, or may be supplied by thedifference amplifier 20 and power amplifier 22 shown in FIG. 2. If thatform of the present invention depicted in FIG. 2 is used, the twoamplifiers, being adjusted to supply filament current generallyinversely proportional to the magnitude of the suppressor ring 11current, will automatically apply a maximum current since the tube 3 istie-energized. When heated in the foregoing manner, the sleeve 27 willabsorb and retain (unless heated to temperatures approaching the meltingpoint of the metal) substantially all such permanently absorbableimpurities while also simultaneously emitting deuterium. Now, if thetube 3 is energized and the deuterium atmosphere 5 is too dense, thecurrent in the suppressor ring 11 circuit will rise above itscharacteristic optimum which may be, for example, about one microamp.This will cause the difference amplifier 20 and power amplifier 22 toreduce the current to the heating filament 28 proportionately, to reducethe temperature of the sleeve 27 and thereby curtail the deuteriumdesorption into the tube atmosphere 5 (or commence absorption ifnecessary) to adjust the pressure as desired. (It is desirable tomaintain at least a part of the titanium sleeve 27 at approximately 900centigrade to remove impurities occasionally appearing.) If the currentto the heating filament 28 is supplied by a controllable source such asthat shown in FIG. 5, then the logging operation will, of course, selectthe proper current in accordance with the pressure indication providedby the suppressor ring lll current as shown by the indicator 15. Aspreviously described and explained, the filament 28 required tostabilize the deuterium atmosphere 5 at the desired optimum pressure(assuming no extremes in ambient temperature) will be the pre-selectedreference current previously mentioned. For a tube 3 and sleeve 27 ofthe geometry heretofore described, this reference current is preferably220 milliamps at about 23 volts. At this point, the sleeve 27 willcontinue to absorb impurities as they occur, and will continue tosimultaneously absorb and desorb deuterium in equal volumes in a stateof equilibrium as long as the tube atmosphere 5 remains at thepreselected optimum pressure. If the pressure drops, a related decreasein suppressor ring 11 current will either indicate to the loggingoperator that he should increase the filament 28 current, or if theinvention is in the form shown in FIG. 2, the ring 11 current decreasewill cause the difference and power amplifiers 20 and 22 toproportionally increase the filament 28 current. Of course, the averageacelerator tube 3 is so designed that the occurrence of impurities willcause a change in the magnitude of the suppressor ring 11 current eventhough the tube atmosphere 5 pressure remains at optimum. Thus, the ring11 current should preferably be compared with a pressure measurementfrom another indicator in order that the cause of a drop in neutronoutput may be correctly identified. However, since the sleeve 27continuously absorbs impurities any changes in suppressor ring 11current may safely be attributed to changes in pressure of the deuteriumatmosphere 5.

Another form the present invention, which has been found extremelyuseful for most logging purposes, is the use of the apparatus shown inFIGS. 3 and 4 together with a constant current power supply whichapplies the pre-selected reference current to the heating filament 28.With a continuous current of this magnitude, the sleeve 27 will beconstantly maintained at the temperature established by the previouslydescribed processing to establish equilibrium of deuterium sorption atthe pre-selected optimum atmosphere 5 pressure. Assuming no tube 3malfunctions due to other causes, the only significant changes inoperating conditions will be changes in ambient temperature in theborehole. However, since the pre-selected reference current appliedcontinuously to the heating filament 28 is of a magnitude such that thesleeve 27 is always at a gettering temperature, only the most extremeambient temperatures will produce a significant change in thetemperature of the sleeve 27.

It should be understood that, except in the case of a deenergizedaccelerator tube 3, the suppressor ring 11 current will never disappearentirely no matter how pure the deuterium atmosphere 5 may become, andno matter how closely optimum pressure may be approached. Moreover, thefluctuation of the ring 11 current is only proportional to thefluctuation of the pressure over a reasonable operating range.Therefore, in that form of the present invention chosen for purposes ofillustration in FIG. 2, the difference amplifier 20 should be adjustedto provide filament 28 current having a maximum and minimum limit. Forequipment and apparatus having the aforementioned dimensions andgeometry, convenient limits have been established at and 300 milliampsat about 15 volts to 35 volts.

The precise shape of the titanium sleeve 27 is not significant to theconcept of the present invention. However, it should be emphasized thatsleeve 27 shape and wall thickness, heating filament 28 shape anddisposition in or next to the sleeve 27, and the magnitude of currentapplied to the heating filament 28, are all factors governing thesorption characteristics of the sleeve 27. As hereinbefore stated, it ispreferable that the sleeve 27 have generally a uniform wall thickness.Thus, a heating filament 28 disposed therein as shown in FIG. 4 willtend to apply heat uniformly along the length of the sleeve 27. Due atleast partly to the conductive action of the support brackets 26, thiswill tend to create a temperature gradient along the length of thesleeve 27, wherein the hottest zone will be generally in the centersection. The temperature at this hot section of the sleeve 27 should bepreferably that sufficient to absorb those atmospheric constituentsother than hydrogen and its isotopes (approximately 900 C.), and shouldlessen progressively in each direction as the ends are approached. Thus,stabilization of the tube atmosphere 5 will be at least partly due tothe cycling action which tends to occur under optimum conditions,wherein the deuterium tends to enter the sleeve 27 ends in greateramounts than it tends to enter the warmer sections, and to flow alongthe sleeve 27 from cooler to warmer areas, where it tends to leave theseareas in amounts greater than it enters. At the sought-for tube pressurethe cycle will be in balance and, small changes in tube pressure may bemore easily corrected by the resulting unbalance. Obviously this is agreat improvement, performance-wise, over a uniformly heated sleeve 27,or over the prior art which utilized two uniformly-heated sorptionfilaments (instead of sleeves 27) wherein one filament is saturated tocapacity and the other is outgassed. In such an arrangement the pressuresensing and signalling apparatus is increasingly complex, and can neverbe dispensed with as in that form of the present invention wherein theheating filament 28 is subjected to merely the constant preselectedreference current hereinbefore described.

It should be understood that the term permanently absorbable, as usedherein in reference to tube atmospheric impurities other than hydrogenand its isotopes, is intended to be used in a relative sense. It is wellknown that there are processes available for removing socalledpermanently absorbed'impurities from the metal. However, since it isneither desirable for necessary to use such processes for practicing orusing the present invention, the impurities may be consideredpermanently absorbed.

Although the forms of the present invention, as depicted and describedherein, are directed to the control of an accelerator tube having adeuterium atmosphere, it is obvious that the present invention iscapable of a much wider application. Not only is it suitable for use inany type of high vacuum electric discharge tube having an internalatmosphere composed of any isotope of hydrogen, but it may be embodiedin many variations of the apparatus and methods described herein. Forexample, the sorption reservoir may be heated directly by the currentnow applied to the heating filament 28 circuit. Numerous othervariations and modifications may be made without departing from theprinciple of the invention. Accordingly, it should be understood thatthe forms of the invention depicted and described herein areillustrative only, and are not intended to be the limits of the presentinvention.

What is claimed is:

The method of manufacturing an electric discharge tube having aninternal atmosphere composed substantially of only a preselectedhydrogen isotope and established at a preselected pressure, said methodcomprising heating a metallic body to a temperature sufficient tosubstantially de-gas said body, cooling said de-gassed body in asubstantially pure bath of said isotope whereby said body is saturatedwith said isotope, disposing said saturated body in the internalatmosphere of said tube, simultaneously heating said tube and exchangingthe contents of said atmosphere with substantially equal quantities ofsaid isotope whereby said tube is de-gassed and said internal atmosphereis formed of substantially only said isotope, thereafter establishingsaid atmosphere at a preselected pressure such that said atmospherecontains substantially less isotope than said body while heating saidbody to a preselected temperature such as to exchange substantiallyequal quantities of said isotope with said internal atmosphere at saidpreselected pressure, sealing said body and said atmosphere in saidtube, and heating said tube to further de-gas said tube while heatingsaid body in said tube to said preselected temperature.

References Cited by the Examiner UNITED STATES PATENTS 2,934,392 4/1960De Santis et a1 SIG-25 FRANK E. BAILEY, Primary Examiner.

